Problems With Evaluation of Word Embeddings Using Word Similarity Tasks
Summary
Word similarity benchmarks are the most common intrinsic test for word embeddings. Models are scored by the Spearman correlation between cosine similarities of word pairs and human ratings. They are cheap and fast, but the authors argue they are unreliable. This workshop paper is a position and survey piece rather than an experimental study. It lists the main weaknesses of this evaluation style and collects the remedies proposed in earlier work:
- unclear and subjective definitions of "similarity"
- a mismatch with task-trained embeddings
- no fixed train/dev/test splits, which encourages overfitting
- weak correlation with downstream results
- missing significance testing
- frequency and hubness effects in cosine space
- polysemy ignored in the gold ratings
The authors note that word analogy evaluation reduces to combinations of word similarities, so the same criticisms apply to it. They conclude that similarity benchmarks can lead researchers to wrong conclusions. Until better intrinsic methods exist, they recommend comparing embeddings on the downstream task of interest. This matters because much embedding research ranks models mainly by these small benchmarks.
Key points
- The surveyed benchmarks are RG (65 pairs), MC (30), WS-353, YP-130, MTurk-287, MTurk-771, MEN (3000), RW (2034), Verb (144) and SimLex (999). Their average size is only about 781 pairs, which makes further splitting unstable.
- Similarity is often mixed up with relatedness. In WS-353, cup–coffee gets a higher rating than car–train, and MEN has similar anomalies. SimLex and the similarity/relatedness split of WS-353 by Agirre et al. try to address this.
- On significance testing, Shalaby and Zadrozny applied Steiger's test, but it needs the full model rankings. Rastogi et al. proposed MRDS, the smallest gap in correlation needed for a result to count as significant, and it can be computed from the correlation values alone. They also showed that many reported gains on small datasets were not significant.
- Schnabel et al. found a power law between a word's frequency rank and the average rank of its 1000 nearest neighbours (exponent ≈ 0.17). As a result, words of similar frequency look artificially similar under cosine.
- Most benchmarks assume one sense per word; for example, WS-353 rates bank–money 8.5/10. SCWS addresses this by rating word pairs inside their contexts.
Interesting ideas
- I1. Cross-benchmark tuning: tune hyperparameters on one benchmark and report results on all the others, following Faruqui and Dyer. This gives held-out evaluation even when no official splits exist, and works for any field with many small test sets.
- I2. Selecting for breadth: pick the configuration with the best average across many tasks, or the one that beats baselines on most tasks (Lu et al.). This favours generalisation over leaderboard tuning on a single dataset.
- I3. Significance thresholds per dataset: publish, for each dataset size, the minimum correlation gap that counts as significant (the MRDS idea). Readers can then judge claimed improvements without access to the raw model outputs.
- I4. Checking for frequency confounds: before trusting nearest neighbours or similarity scores, test whether neighbours mostly share frequency rank rather than meaning. This check is useful for any embedding-based retrieval, not only for benchmarks.
- I5. Grading embeddings by their training objective: do not judge embeddings learned for one task, such as POS tagging, by a semantic yardstick. Choose evaluation that matches what the vectors were trained to encode.
Cited by
- Morphological Word Embeddings for Arabic Neural Machine Translation in Low-Resource Settings
- Analysis Methods in Neural Language Processing: A Survey
- Interpreting Pretrained Contextualized Representations via Reductions to Static Embeddings
- SentEval: An Evaluation Toolkit for Universal Sentence Representations
- SimVerb-3500: A Large-Scale Evaluation Set of Verb Similarity
- Where New Words Are Born: Distributional Semantic Analysis of Neologisms and Their Semantic Neighborhoods
- Intrinsic Bias Metrics Do Not Correlate with Application Bias
Cite
@inproceedings{faruqui2016problems,
title = {{Problems With Evaluation of Word Embeddings Using Word Similarity Tasks}},
author = {Faruqui, Manaal and Tsvetkov, Yulia and Rastogi, Pushpendre and Dyer, Chris},
booktitle = {Proceedings of the 1st Workshop on Evaluating Vector-Space Representations for NLP},
year = {2016},
doi = {10.18653/v1/w16-2506},
url = {https://doi.org/10.18653/v1/w16-2506}
}References
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